Method of making a lead frame for integrated circuits

ABSTRACT

A lead frame for integrated circuits has a central opening in which there is fitted a metal piece having alternate recesses and projections, the recesses fitting over inner ends of leads of the lead frame and the projections fitting in gaps between the inner lead ends. The metal piece includes slots and cut-outs to allow its comb-shaped members to bend independently of each other. After the lead frame has been fused to a ceramic substrate, the metal piece can be removed by being pulled upwardly with a relatively small force applied. The metal piece has a pair of pull tabs. For fabricating such a lead frame, the metal piece is stamped out of an integral metal plate structure having the leads and support frame integral with the leads. Then, the stamped metal piece is pushed back so as to fit with the leads of the lead frame.

This application is a divisional application of application Ser. No.425,102, filed Sept. 27, 1982, now U.S. Pat. No. 4,523,218.

BACKGROUND OF THE INVENTION

The present invention relates to an improved construction of a leadframe for integrated circuits (IC) and a method of fabricating such thelead frame.

A lead frame for use in a glass-encapsulated IC package has a centralopening and a multiplicity of leads with free inner ends. Since theinner ends are not securely supported, they tend to bend upwardly ordownwardly, or laterally. The bent lead ends cannot be bondedsufficiently to a ceramic substrate coated with glass, or securelybonded to aluminum wires.

One known proposal to eliminate the above difficulties is described inU.S. Pat. No. 4,137,546 to Frusco. The disclosed lead frame has aplurality of break-off tabs integral with the inner lead ends. Thebreak-off tabs are severed off the inner lead ends after the leads havebeen bonded to the substrate. However, the inner lead ends can still bebent since the break-off tabs are independently movable. Removal of thebreak-off tabs cannot be mechanized since they will be cut off by beingbent up and down several times.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lead frameconstruction having all inner lead ends which remain completelysupported until they are bonded to a glass-coated ceramic substrate andwhich are releasable of such support in a single operation after theyhave been bonded to the ceramic substrate.

Another object of the present invention is to provide a method offabricating such a lead frame construction.

According to the present invention, a lead frame for use in an ICpackage has a multiplicity of leads extending outwardly from a centralopening and having inner and outer ends, and a support frame integrallysupporting the outer ends of the leads. A metal piece is disposed in thecentral opening and has in peripheral edges thereof alternate recessesand projections, the recesses fitting over the inner ends and theprojections fitting in gaps between the inner ends. The metal piece alsohas slots spaced inwardly from the recesses and projections and cut-outsdefined between the slots. The slots and the cut-outs jointly divide themetal piece into a plurality of comb-shaped members interconnected by aplurality of bendable portions defined therebetween by the slots and thecut-outs, the comb-shaped members being independently bendable.

To fabricate the lead frame, a metal plate structure is prepared whichhas a portion to be severed later as the metal piece, a multiplicity ofleads extending outwardly from such portion, and a support frameintegral with outer ends of the leads. The portion is stamped as themetal piece out of the metal plate structure to allow all of inner endsof the leads to project independently of each other, the stamped portionhaving alternate recesses and projections in its peripheral edges. Thestamped metal piece is pushed back until the recesses fit over the innerends of the leads and the projections fit in gaps between the inner endsof the leads.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiment of the present invention is shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a conventional lead frame for use in adual-in-line ceramic package;

FIG. 2 is a perspective view of a lead frame according to the presentinvention;

FIG. 3(A) is an enlarged scale plan view of a metal piece to be mountedin the lead frame shown in FIG. 2;

FIG. 3(B) is a front elevational view of the metal piece illustrated inFIG. 3(A);

FIG. 4 is a front elevational view of the metal piece of FIG. 3(A) as itis pulled;

FIGS. 5(A) and 5(B) are cross-sectional views illustrative of apush-back step in a lead frame stamping operation.

DETAILED DESCRIPTION

FIG. 1 shows a known lead frame for use in a ceramic package comprisinga semiconductor integrated circuits device (hereinafter referred to asan "IC chip") mounted on a ceramic substrate. The lead frame has acentral opening 1 and includes a multiplicity of leads 2 extendingoutwardly from the central opening 1 and integral with a support frame3. The lead frame is used for a dual-in-line ceramic package(hereinafter referred to as a "Cer-Dip") and is fabricated by stamping aweb of Fe-Ni alloy coated with strips of aluminum (not shown) onprogressive dies, and bending outer lead end portions away from thealuminum-coated surface into a channel shape.

The Cer-Dip is assembled as follows: A ceramic substrate 4 having alayer of gold on its surface portion positionally corresponding to thecentral opening 1 in the lead frame and a glass layer 5 of a low meltingpoint which covers the upper surface of the ceramic substrate 4 arebonded to each other by heating, the IC chip is fixed to the gold layerin the central opening 1, and then aluminum wires are bonded to the ICchip and to the leads 2. Thereafter, a cover (not shown) identical inshape with the ceramic substrate 4 and having a glass layer of a lowmelting point on its surface for contact with the lead frame, is placedon the lead frame and heated. The lead frame is now sandwiched andhermetically sealed with heat between the ceramic substrate 4 and thecover. Finally, the support frame 3 of the lead frame is cut off tocomplete the Cer-Dip.

With the lead frame of this configuration, the leads 2 are supported attheir outer ends by the support frame 3 but not supported at their innerends. The inner lead ends are thus apt to bend upwardly or downwardly,or laterally during shipment or while the lead frame is being bonded tothe ceramic substrate 4. When the inner lead ends are turned upwardly,they cannot be fused in position by the glass layer 5. When the innerlead ends are bent laterally, they may be brought into mutual contactresulting in bonding failure or malfunctions of the completedsemiconductor device.

The inner lead ends are apt to bend more frequently the more leads thereare and as the leads become narrower and longer. One proposal to reducesuch a disadvantage is disclosed in the aforementioned U.S. Pat. No.4,137,546 to Frusco in which tabs supporting inner ends of a pluralityof leads can be broken off. With this construction, there is no problemfor the inner lead ends to be deformed before the leads are fused to theceramic substrate and hence the leads are less subjected to unwantedbending.

However, the prior arrangement is disadvantageous in that the break-offtabs, two to four in number, may each cause bending of all of the leadends connected thereto. The break-off tabs are severed from the leadends by being bent up and down several times, a procedure which isdifficult to mechanize.

The present invention will now be described in detail with reference tothe drawings.

FIG. 2 shows a lead frame according to the present invention. The leadframe of FIG. 2 includes a metal piece 6 fitted in the central opening 1in the lead frame as shown in FIG. 1 with the inner ends of the leads 2being fastened temporarily to the metal piece 6.

FIGS. 3(A) and 3(B) illustrate the metal piece 6 on an enlarged scale.The metal piece 6 has alternate recesses 7 and projections 8 in itsperipheral edges. The recesses 7 serve to receive the inner ends of theleads 2 and the projections 8 serve to extend into gaps between theadjacent inner lead ends. The metal piece 6 also has slots 9 extendingparallel to the peripheral edges thereof and inwardly of the recesses 7and projections 8, and cut-outs 10 defined respectively in the fourcorners of the metal piece 6 and extending substantially in diagonaldirections of the metal piece 6, thus providing four comb-shapedmembers. A pair of opposite pull tabs 11 are bent from a central portionof the metal piece 6 and arranged symmetrically with respect to a centerline of the metal piece 6.

When the pull tabs 11 are pulled upwardly, the stresses are concentratedon four bendable portions 12 defined between the slots 9 and thecut-outs 10 to bend the bendable portions 12 with the four comb-shapedmembers being independently of each other as shown in FIG. 4. Thus, themetal piece 6 can smoothly be removed. If there were no slots 9 andcut-outs 10, it would require an undue force to remove the metal piece 6tending to break the glass layer 5 as shown in FIG. 1 or raising theleads 2 off the glass layer 5. For example, with a metal piece for usein a lead frame having 40 leads, a force of about 10 Kg was needed toremove the metal piece when the metal piece had no slots 9 and cut-outs10. When the metal piece had slots 9 and cut-outs 10, the metal piececould be removed with a force of about 4 Kg applied. If, however, themetal piece could be removed with too small a force, then the metalpiece would come off when subjected to a small shock. To cope with thisproblem, the depth of the recesses 7 defined in the peripheral edges ofthe metal piece 6 may be changed to adjust the force with which themetal piece 6 is supported or can be removed. The depth of the recesses7 should preferably be 0.1 to 4 times the width of the inner ends of theleads 2, and should more preferably be 0.5 to 1 times that width. Theslots 9 should be spaced from the recesses 7 by a distance which is twoor more times the thickness of the metal piece 6. The slots 9 andcut-outs 10 should have a width that is about 0.5 to 1.5 times thethickness of the metal piece 6. The ends of the adjacent slots acrosseach bendable portion 12 should be spaced from each other a distancewhich is substantially equal to the thickness of the metal piece 6. Thepull tabs 11 may be of any desired shape provided they can be pulledupwardly. The pull tabs 11 as shown in FIG. 3 have aligned central holes13, respectively, through which a rod or the like can be inserted tolift the metal piece 6. Thus, removal of the metal piece 3 can easily bemechanized.

The lead frame having such a metal piece 6 can easily be fabricated bystamping. More specifically, a sheet of metal is etched and stamped toform an integral metal plate structure including the metal piece 6, theleads 2 and the support frame 3. The metal plate structure is stamped todefine the pull tabs 11, the slots 9 and cut-outs 10. Then, the metalpiece 6 is stamped out of the metal plate structure so as to allow allof inner ends of the leads 2 to project independently of each other. Thestamped metal piece 6 is then "pushed back" to cause the recesses 7 andthe projections 8 to fit over the inner lead ends and in the gapstherebetween. The above fabrication procedure can easily be carried outin a pressing operation using progressive dies.

As shown in FIGS. 5(A) and 5(B), the metal piece 6 is first stamped outof the metal plate structure (FIG. 5(A)) and then pushed back to itsoriginal position (FIG. 5(B)), a process which is known in the field ofstamping operation. The pull tabs 11 should preferably be bent upwardlyafter the metal piece 6 has been pushed back.

A lead frame for use in the Cer-Dip can normally be stamped out of ametal web on progressive dies. The lead frame according to the presentinvention can be fabricated on such progressive dies with the abovepush-back step added.

While the present invention has been described as being applied to leadframes for use in the Cer-Dip, the invention is not limited to such anapplication. For example, the present invention is applicable to a leadframe for a flat package having outer lead end portions extendingflatwise or not bent in a channel shape, or to such a lead frame whichhas inner lead ends plated with gold, silver or the like. The inner leadends may be shaped in a variety of configurations such as rectangular,triangular, or semicircular forms.

With the arrangement of the present invention, highly reliable packagesin which a ceramic substrate and a lead frame are bonded with glass canbe fabricated with an improved rate of yield.

Although a specific preferred embodiment has been shown and described,it should be understood that many alterations and modifications may bemade therein without departing from the scope of the appended claims.

What is claimed is:
 1. A method of fabricating a lead frame for use inan integrated circuit package, comprising the steps of:preparing anintegral metal plate structure which includes a support frame, a centralsection, a multiplicity of spaced lead elements extending between saidcentral section and said frame, the outer ends of said lead elementsbeing integrally connected with said frame and the inner ends of saidlead elements being integrally connected with said central section alongits periphery, said central section having slots spaced inwardly of saidperiphery and cutouts defined between said slots; severing a portion ofsaid central section from said lead elements to provide a metal piecewhose periphery includes alternate recesses and projections, saidrecesses being formed at locations where the inner ends of said leadelements were previously connected to said central section, said slotsand said cutouts jointly dividing said metal piece into a plurality ofcomb-shaped members interconnected by a plurality of bendable portionsdefined therebetween by said slots and said cutouts, said comb-shapedmembers being independently bendable; moving said metal piece in a firstdirection with respect to said lead frame so as to separate the recessesin the periphery of said metal piece from the extended tips of said leadelements; and moving said metal piece in a second direction oppositesaid first direction so as to reposition the recesses in the peripheryof said metal piece around the respective extended tips of the leadelements.